The present invention pertains to monolithic catalyst supports having a cellular or honeycomb structure.
Such catalyst supports are becoming increasingly useful in stationary emissions control of power plants and chemical synthesis and process facilities. A catalyst structure similar to that of the invention is usually fabricated by washcoating a monolithic support using techniques developed for the automotive industry.
A catalyst structure can also be fabricated by extruding a substantially homogeneous cellular ceramic composed of the catalyst and substrate materials. Alternately, similar extruded honeycombs can be impregnated with catalyst precursor compounds after fabrication, and then heat treated. Such structures are now being utilized in Japan and West Germany for NO.sub.x conversion.
Conventional washcoating techniques generally produce a coating of high surface area oxide in combination with one or more catalysts.
In automotive converters, the catalysts usually employed are noble metals, such as platinum, palladium and rhodium. These noble metals in the form of processor chemicals are coated upon the ceramic honeycomb support with high surface area support metal oxides, such as alumina and ceria, that are incorporated into the washcoat as oxides, precursor chemicals or mixtures.
The catalytic precursor chemicals are used with the washcoat at either an initial stage or a later stage of the process in order to provide good dispersion of the ultimate catalyst particles, which are usually in low concentration (e.g., less than one weight percent) in the solution. After washcoating, the coated support is dried and heated in a controlled atmosphere in order to convert the precursor chemicals to the phase appropriate for the intended catalytic operation.
By contrast to the conventional washcoats, however, the present invention uses metal powders, alloys and mixtures as precursors for eventual conversion into catalytic metal oxides/sulfides for dispersion on metal oxide support materials constituting the washcoat.
Also, the present invention not only converts the precursor metal powders to catalytic oxides/sulfides, but also greatly reduces their particle size after they have been washcoated upon the substrate. This unique step provides many advantages, one of which is the use of larger particles for catalyst metal precursors in the washcoat as compared to the ultimate catalyst particle size. The utilization of the large particles allows for ease of separation and recovery of expensive or toxic metal catalyst precursor particles in the waste solution. Also, the larger particles provide an easier handling of toxic metals, since harmful, small particles that cause dust, which can be injested or which can lodge in the pores of the skin, are avoided.
Another advantage of the invention is the one step washcoating procedure.
Another advantage of this invention is the ability to incorporate multiple catalyst precursors into the washcoat in one step, as compared with the multiple step procedures now commonly employed with conventional washcoating processes.
The conversion of the metals is accomplished by post fabrication treatments. For example, the catalyst and support metal precursors can be oxidized in situ by heat treating, or they can be utilized in conjunction with oxidizing or sulfiding heat treatments to oxidize or sulfidize them after the washcoat has been applied.
Enough metal precursor powder can be incorporated into the washcoat so that reserve metal can be further activated by further oxidation in situ, after the initial catalyst has lost potency with use.
The washcoats of this invention can be applied to ceramics, glass, metal or composite forms, including honeycombs and multicellular substrates.